The present invention relates generally to the art of transmitter apparatus and more particularly to systems and methodologies for calibration of transmitter systems.
In broadband and wireless communications systems, information is transferred between sources and destinations through various communications media. For example, in wireless transmitter systems, analog signals carrying information are transmitted to a communications media such as the ambient via an antenna. In digital systems, the signal to be transmitted is generated initially by a digital processing system and is provided to a modulator system, where it is converted to an analog baseband signal and filtered to remove frequency components associated with the digital to analog conversion. The filtered signal is then provided to one or more mixers with associated local oscillators in the modulator system, which perform frequency translation to provide a radio frequency (RF) or intermediate frequency (IF) signal. For example, two or more such mixers may be cascaded to initially up convert an initial signal of between DC and 1 MHz to an intermediate frequency, such as 300 MHz. Thereafter, a second mixer converts the IF signal to an RF signal, such as at about 2 GHz, which is then amplified for transmission via an antenna. One or more signal conditioning circuits, such as amplifiers and/or filters, may be provided between successive mixer stages in the modulator system. Alternatively, a single modulation is performed, by which the baseband signal is modulated directly to an RF transmission frequency.
Recently, the frequencies of signals within such circuits are increasing in order to provide improved speed, conforming to communication protocols or standards, etc. Consequently, analog signal chains in such transmitter systems and the modulators thereof in modern communications systems are required to operate at ever higher frequencies with larger dynamic range, decreased distortion, improved carrier suppression, and at lower power supply voltages. In order to improve signal fidelity along the analog signal chain of transmitter modulator circuits, it is desirable to improve carrier suppression by reducing or otherwise compensating for DC offsets and/or signal leakage associated with the various components therein.
Thus, for example, in conventional digital transmitter designs, DC offsets associated with the digital to analog converter (DAC) and the low pass filter (LPF) are sometimes calibrated to improve carrier suppression. While such calibration has heretofore provided some measure of carrier suppression improvement, further reduction in system DC offsets is desired to facilitate and enhance transmitter system performance. Conventional digital transmitter systems perform a calibration to determine DC offset values for the DAC and LPF. The output of the low pass filter stage is fed back to an analog to digital converter, and an offset value is determined based on application of DC voltages to the LPF using the DAC. Subsequent signal information provided to the DAC during normal operation is offset by the offset value determined during calibration.
However, other offsets exist in typical transmitter modulator circuits, which are not taken into account in conventional modulator calibration apparatus and methodologies. For instance, DC offsets may be found in one or more mixer stages within a transmitter signal chain, which are not accounted for in current calibration techniques. Such mixer offsets result from difficulties in matching components within the mixer circuitry during fabrication of the mixer components and from leakage in the mixer circuitry. Furthermore, offsets may exist in the calibration feedback circuitry, which may inappropriately skew the calibration offset value.
For instance, Gilbert cell circuits are often employed as mixers for transmitter modulators fabricated using bipolar or BiCMOS processes. Offsets may result in Gilbert cell or other type mixer circuits, due to imperfect matching of resistors and/or transistors in the cell. Such component mismatch, for example, may allow leakage of local oscillator (LO) signal currents into the RF output of the mixer, and cause offsets in the system. This signal leakage must be minimized or reduced, or otherwise accounted for in order to attain optimal or improved carrier suppression in such devices. In this regard, Gilbert cell type mixers typically include a transconductance amplifier generating differential current signals corresponding to a baseband (e.g., or IF) input signal thereto. Mismatches in the components of the transconductance amplifier can cause leakage of the LO signal into the RF output of the mixer. In addition, mismatching of other components in the Gilbert cell type mixer, such as transistors and resistors apart from the transconductance amplifier, can also contribute to offsets in the mixer. Thus, there remains a need for improved apparatus and methodologies by which improved carrier suppression may be achieved in transmitter modulator circuits.
The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. The invention provides systems and methodologies for calibrating transmitter circuits and devices, by which the above and other shortcomings associated with the prior art are mitigated or overcome.
Methods and systems are provided for calibrating transmitter systems, such as including Gilbert cells and other types of modulators, in which mixer nodes associated with a local oscillator (LO) input are held at certain voltages while a DC signal is provided as a baseband signal and the RF output of the transmitter mixer is monitored to ascertain a polarity associated therewith. The DC signal is then changed, such as by successively sweeping through a variety of DC values, until a polarity change is detected at the mixer RF output. The DC value at which the polarity change occurred is saved as a first offset value. The first offset value may then be used as a calibration offset factor during normal transmitter modulation operation, whereby offset effects of the mixer are compensated or accounted for in modulating baseband signals for transmission.
Alternatively or in combination, a second offset value may be determined, wherein the first and second offset values are used to derive a calibration offset value for use in normal operation. In this regard, following determination of the first offset value, the mixer nodes are held at different voltages, and the process is repeated to determine a DC value at which the mixer RF output again changes polarity, which is then saved as a second offset value. A calibration offset value for the transmitter system is then determined according to the first and second offset values, such as by averaging. The resulting calibration offset value is then added into subsequent baseband signals (e.g., either digitally or in analog form) during subsequent transmitter operation.
Because the calibration utilizes the mixer RF output as feedback, the offsets and leakage associated therewith are taken into account in the final calibration value. For instance, DC leakage from the LO input to the RF output of the mixer is factored into the calibration, as are component mismatch related offsets in the mixer. In addition, where the baseband signal path involves a digital to analog converter (DAC) and associated low pass filter (LPF) stage, offsets therein are also accounted for in the calibration offset value determined according to the invention. Furthermore, the invention provides for cancellation of offsets in the calibration feedback circuit in determining the calibration offset value. Thus, whereas prior calibration techniques introduced such feedback amplifier offset errors into the calibration factor, the present invention avoids this source of error by providing the calibration offset value based on first and second offset value determinations.
According to one aspect of the invention, a method is provided for calibrating a transmitter system having a mixer with a local oscillator (LO) input, a baseband input, and an output. The method comprises applying first and second voltages to first and second nodes associated with the local oscillator input, respectively, wherein the second voltage is greater than the first voltage, and determining a first offset value corresponding to a voltage applied to the baseband input at which the output is about zero. The first offset value may then be saved as a calibration offset value or factor for use in normal operation. Alternatively, third and fourth voltages are applied to the first and second nodes, wherein the third voltage is greater than the fourth voltage, and a second offset value is determined corresponding to a voltage applied to the baseband input at which the output is about zero. In this case, the method further comprises providing a calibration offset value according to the first and second offset values, for example, by averaging.
Another aspect of the invention provides a calibration system for calibrating a transmitter modulator having a mixer with a local oscillator input for receiving a local oscillator signal, a baseband input for receiving a baseband signal from a low pass filter, and a mixer output. The calibration system comprises a first system, such as a digital system providing a DC signal to the low pass filter during calibration and a second system, such as a feedback amplifier and comparator receiving the mixer output and providing an indication of a polarity associated with the mixer output signal to the first system during calibration. A third system individually applies voltages to first and second nodes associated with the local oscillator input during calibration, and the first system determines a first offset value corresponding to a voltage applied to the baseband input at which the mixer output is about zero when first and second voltages are applied to the first and second nodes, respectively. The first system also determines a second offset value corresponding to a voltage applied to the baseband input at which the mixer output is about zero when the third system applies third and fourth voltages to the first and second nodes, respectively. The first system then calculates or otherwise derives a calibration offset value according to the first and second offset values, such as by averaging.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.